1. Field of the Invention
The present invention relates generally to hyperthermia devices, and more particularly, to hyperthermia devices utilizing induction heating.
2. Description of the Background Art
Solenoids have been commonly used as magnetic applicators for induction heating in hyperthermia. A magnetic applicator comprising a magnetic core such as ferrite and a coil winding around the same is particularly preferable to a magnetic applicator including only a solenoid in that it is capable of locally concentrating electric field. In induction heating in general, however, eddy current density tends to be high in a peripheral portion of a body to be heated and it is therefore difficult to selectively heat a desired local region in the body. In particular, it is difficult to heat a portion located deep in the body.
FIG. 1A located schematically shows a main portion of a conventional hyperthermia device and FIG. 1B shows a section taken along the chain dotted line of FIG. 1A. The hyperthermia device comprises a pair of magnetic applicators 1a and 1b arranged to have a body 4 to be heated provided therebetween. The magnetic applicators 1a and 1b include magnetic cores 2a and 2b, and coils 3a and 3b, respectively. An alternating field is applied to the body 4 by a flow of the alternating currents of the same phase to the coils 3a and 3b of the magnetic applicators 1a and 1b, respectively. The alternating field generates an eddy current in the body 4 which will be heated by Joule heat generated by the eddy current.
The hyperthermia device shown in FIG. 1A is designed to have the central portion of the body 4 through which the common axis of the magnetic cores 2a and 2b, that is, the central portion of the section 4a of FIG. 1B heated. As described above, however, the eddy current density tends to be high in the peripheral portion of the body 4 and consequently, there is a tendency that only the peripheral portion of the section 4a is heated while the central portion of the same is not heated. Then, as shown in FIG. 1B, hot spots 4b might be produced in the peripheral portion of the body 4.
FIG. 2A shows the distribution of eddy current densities in the section 4a obtained as a result of simulation performed by the finite element method using, phantom equivalent to muscle as the body 4 shown in FIG. 1A. The phantom 4 has an electric conductivity of 0.62S/m and a dimension of 30.times.30.times.30 cm.sup.3. An alternating field having a frequency of 5 MHz is applied to the phantom 4 and the line of magnetic force runs perpendicular to the surface of the drawing FIG. 2A. Each vector shown in the phantom 4 has a length proportional to an eddy current density and the vector shown at the lower right of FIG. 2A corresponds to an eddy current density of 1.2 kA/m.sup.2. It is clear from FIG. 2A that the eddy current density is high in the peripheral portion of the phantom 4, in particular, in the regions corresponding to the hot spots 4b shown in FIG. 1B. In the drawing, the axis x represents a horizontal position of the phantom 4 and the axis y represents a vertical position thereof.
FIG. 2B shows electric power loss at positions along the line 2B--2B of FIG. 2A. The line 2B--2B crosses the phantom 4 at a height of y=15 cm in a direction along the axis x. The electric power loss is electric energy consumed for generating the Joule heat, and the larger the electric power loss of a position is, the more the temperature of the position increases. As is clear also from FIG. 2B, the central portion of the section 4a of the phantom 4 is barely heated as compared with the peripheral portion thereof.
As described above, the prior art hyperthermia device has difficulty in selectively heating a desired local region of a body to be heated, in particular, to heat a portion located deep in the body.